Stabilization of tuned radio frequency amplifiers



p 0- E. F. w. ALEXANDERSON 1,775,544

STABILIZATION OF TUNED RADIfiEQUENCY AMPLIFIERS Filed May 9, 1928 Fig.1

ION:

' His Attorney Patented Sept. 9, 1930 UNITED STATES PATENT. OFFICE ERNST F. W. ALEXANDERSON, OF SCHENEGTADY, NEW YORK, ASSIGNOR TO GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK STABILIZATIONOF TUNED RADIO FREQUENCY AMPLIFIERS Application filed May 9,

The present invention relates to tuned radio frequency amplifiers and more particularly to an improved means for stabilizing the operation of such amplifiers.

5 Amplifiers of this character usually employ space discharge devices of the vacuum tube type provided with three or more elements and connected in cascade by suitable tuned radio frequency transformers or impedances.

In the operation of amplifiers of this character, a problem of stabilization arises due to' the fact that each tube reacts upon its grid or tuning circuit in such a way that it introduces the equivalent of a negative resistance therein. This reaction is due to the capacity coupling between the grid and the plate of the tube. If the negative resistance becomes greater than the positive resistance of the grid circuit, the tube will oscillate. In long wave amplifiers this difficulty is not serious because the negative resistance is smaller than the positive resistance. Radio frequency amplifiers of broadcast receivers may or may not have a tendency to oscillate depending upon the proportions of the circuits.

While successful amplifiers have been built which are so proportioned that the tendency to oscillate is avoided by making the negative resistance comparatively small,this arrangement has the disadvantage that a comparatively low degree of amplification is realized. Higher amplification is usually accompanied by a higher negative resistance and therefore a greater tenoency to'oscillate.

One solution of this difficulty is disclosed by Letters Patent of C. W. Rice, No. 1,334,118, patented March 16, 1920, which relate to a method of neutralizing the capacity coupling by a VVheatstone balance. This method is 40 commonly used in receiving sets which are commercially known as the neutrodyne type.

In the past, oscillations in radio frequency amplifiers have been avoided by using'coil systems with inherent high resistance. Thus by artificially increasing the positive resistance it was possible to use proportions of circuits which gave higher amplification and produced a high negative resistance without creating oscillations. It is possible to obtain high amplification in this way at broadcast 1928. Serial No. 276,470.

wave lengths or frequencies, but the difficulty in the use of this method heretofore has been that the negative resistance created by the tubes is much higher at the shorter waves or higher frequencies than at the longer waves or lower frequencies. Therefore, sets designed in this way have a tendency to oscillate at short waves while they are ineificient at long waves.

From these considerations, it is evident that it wouldbe desirable to provide an artificial increase in the resistance of the coils or tuned circuitsin an amplifier for high frequencies graded in such a way that the additional resistance of the coils of circuits would match the negative resistance of the space discharge devices or tubes usedtherewith, at all wave lengths or frequencies. In accordance with my invention an improved stabilizing circuit including an element having a high dielectric hysteresis loss is associated with the coils or tuned circuits of a tuned radio frequency amplifier and is adjusted so as to give this desired result.

In carrying out my invention, I take advantage of a property of a dielectric, such as asbestos, which has a very high dielectric hysteresis constant. In measurements of dielectric hystersis which I have made of he I quencies up to 100,000 cycles, I have found that asbestos has a power factor of about 30% for all frequencies up to 100,000 cycles, and it also appears that it has substantially the same power factor at much higher frequencies. The expression high dielectric constant is used herein to designate any suitable material oi this general character. The high energy absorption of asbestos can be demonstrated by simply placing a piece of asbestos tape on top of the tuning coils of a radio frequency amplifier. If the amplifier is near the critical point of oscillation this alone will stop it.

In order to get the desired gradation of oscillation, I use asbestos or a dielectric having similar properties in a particular way. This can best be described with reference to the accompanying drawing in which Fig. 1 is a diagrammatic representation of the principle on which the stabilizing means of my invention operates; Fig. 2 is a wiring diagram of a tuned radio frequency amplifier provided with a stabilizing circuit in accordance with my invention, and Fig. 3 is a wiring diagram of a similar amplifier provided with a modification of such stabilizing circuit.

Referring to 1, 5 and 6 are plates of conducting material connected with circuit wires '7 and separated by a strip of dielectric material 8 having very high dielectric hysteresis constant with respect to air, for example, such iece of asbestos tape. This serves as an electrostatic coupling between the plates which may be connected with a tuned radio frequency amplifier to form a stabilizing circuit therefor, the stabilizing circuit being completed through the amplifier coil or tuning system for example, and the dielectric between the plates acting as the stabilizing or energy absorbing means. The plates may be taken as representing conducting bodies or portions of a circuit which may be coupled electrostatically by the dielectric 8 to increase the positive resistance of the circuit. One stabilizing circuit arrangement embodying a condenser means of this type is shown in Fig. 2. Referring to dig. 2, 9 is a threeelement vacuum tube connected in the usual manner between two tuned radio frequency transformers 10 and 11, the secondary 12 of the first transformer, together with a variable shunt capacitance 13 forming a tuned impedance or grid circuit therefor, and the primary 14; of

the second transformer forming the output or load circuit therefor. This may be taken as representing any suitable radio frequency or similar amplifier circuit including a tuning coil or tuned circuit connected with a space discharge device which tends to set up osclllatlons in the tuned circuit. In the present example, the oscillations occur by reason of the plategrid capacity coupling of the tube which tends to produce the effect of a negative resistance greater than the positive resistance ofthe grid circuit- The problem as stated hereinbefore is then to provide ameans inconnection with or associated with the grid or tuned circuit, preferably at the tuning coil, or impedance, which will set up an artificial increase in the positive resistance of the circuit to equal or slightly overcome the negative resistance in the tube or other space discharge device connected with the coil circuit, at all wave lengths, that is, at all frequencies over which the tuning system is to operate.

One means for effectingv this result and for carrying out the principles underlying the showing in 1, in connection with the tuned radio frequency amplifier above described comprises a small coil 15 which is wound with two layers 16 and 17, with an asbestos tape 18 between the layers. The tape may, however, be of any other suitable material having similar dielectric properties, and serves as an energy absorbing means in connection with the coil. The natural period of this coil should occur preferably at ahigher frequency than the highest frequency at which the amplifier circuit is to be used. One terminal of this coil is connected through a circuit wire 19 to the ground or low potential end of the tuned cir cuit or secondary 12. The other terminal is connected by a wire 20 to a piece of conducting material 21, such as a piece of tinfoil for example, which may have an area of about square inch and is laid adjacent to the high tension or grid end of the tuned circuit or secondary 12 and-separated therefrom by a piece of asbestos tape 22. v The small absorption coil 15 is preferably coupled magnetically to the tuning coil or tuned circuit by being placed inside it or in front of it, as indicated in the drawing. i

This arrangement provides a stabilizing circuit 19-16-1720-21-22 in connection with the tuned circuit or secondary 12 in which stabilizing circuit is provided an en ergy absorbing or stabilizing condenser comprising the plate 21, the adjacent portion of the winding 12 and the dielectric 22.

In the design of the stabilizing means or circuit, there are a number of variables which can be used so as' to grade the positive resistance of the tuned circuit with reference to the wave length or frequency in any way desired. The principal variables are the inductances of-the absorbing coil 15, the capacity coupling through the stabilizing condenser 21-22 and the magnetic coupling of the stabilizing winding 15 with the grid coil or tuned circuit. v

I have found it advantageous to design the coil and the stabilizing condenser so as to ob tain a little greater absorption than is necessary and then partly neutralize this absorption by adjustment of the magnetic coupling which will be in such a directionthat the magnetic and the electrostatic con pose each other. It ispossible, for to adjust these couplings so that the static and electromagnetic coupling electroexactly neutralize each other at the longest wave or lowest frequency to which the tuned circuit is to respond. Thus there will be no energy absorption in the asbestos tape or dielectric included in the coil and theonly loss will be the loss in the stabilizing condenser.

I If the electrostatic and electromagnetic couplings are thus balanced at the longest wave it can readily be seen that at the short waves the electrostatic coupling is greatly increased whereas the el ctromagnetic coupling remains about constant. a tendency'for voltage to build up across the absorption coil and his voltage in its turn causes energy absorption in the dielectric introduced between the layers. At the wave There will thus be lengths corresponding to the natural period of the absorption coil this energy absorption would be very high.

It is therefore desirable to have the natural period of the absorption coil correspond to a wave length which is sufiiciently shorter than the shortest Working wave length that its frequency resonance point will he higher than the highest working frequency of the amplifier. However, it can readily be seen that a suitable proportioning of the coil with reference to this natural period gives one more variable for increasing the energy absorption of the shortest wave or highest frequency to any desired degree. By'suitably proportioning these separate variables I have found it possible to design an amplifier that is just about at the point of oscillation over the whole range of broadcast wave lengths or frequencies without actually oscillating at any wave length or frequency.

Many practical variations of the principles outlined may of course be used. For example, as indicated in Fig. 3, to which attention is now directed, the absorption coil shown at 23 may be made of asbestos or dielectric covered wire, the dielectric being indicated at 24, and may be wound on the same cylinder the tuning coil or inductance 12, which is shown connected together with tube 9, in the same radio frequency amplifier circuit as in Fig. 1. In this arrangement the spacing of the coils is such as to give the desired capacity and magnetic coupling. In the present example it is indicated as being wound with an asbestos or dielectric covered wire which is fastened on the inside of the cylinder of the tuning coil, at the high tension or grid end, the capacity between the two windings being depended upon for the desired coupling. For this purpose one end of the coil 23 is left open as indicated at 25.

From the foregoing description. it will be seen. that the stabilizing means and circuit of my invention are readily adapted for application to any radio frequency amplifier having a tuning coil or tuned circuit in connection with a space discharge device, the inter element capacity of which provides an undesirable negative resistance effect upon the tuned circuit. In certain cases the stabilizing circuit may include only the dielectric stabilizing condenser of Fig. 1 for electrostatic coupling with the tuned circuit and in other cases may include also means such as the small coil 15 of Fig. 2 for electromagnetic coupling or a single coil such as used at 23 in Fig. 3 for providing both electrostatic and electromagnetic coupling, the coil itself being wound with dielectric covered wire, the dielectric having a high hysteresis constant.

What I claim as new and desire to secure by Letters Patent of the United States is 1. The combination with a radio frequency amplifier including elements providing a tuned circuit and a space discharge device connected with said tuned'circuit, of a stabilizing circuit including a condenser connected with said tuned circuit, the dielectric of said condenser having a high hysteresis constant with respect to air and being closely associated with one element of said tuned circuit.

2. The combination with a radio frequency amplifier including elements providing a tuned circuit and a space discharge device connected with said tuned circuit, of a stabilizing circuit connected with said tuned circuit and provided with a condenser having a dielectric of asbestos and being closely associated with one element of said tuned circuit.

3. The combination of a. tuned radio frequency amplifier comprising a tuned circuit and a space discharge device connected therewith, a stabilizing circuit including a capacity coupling having a high dielectric hysteresis constant, and an'inductance provided with coils which are spaced apart by a dielectric of the same character as the dielectric of said. coupling.

4. The combination of a tuned radio frequency amplifier comprising a. tuned circuit and a space discharge device connected therewith. and a' stabilizingcircuit coupled to said tuned circuit through a'capacity coupling having a high dielectric hysteresis constant, said stabilizing circuit being tuned to a frequency higher than the highest operating frequency of said tuned circuit.

5. The combination of a tuned radio frequency amplifier comprising a tuned circuit and a space discharge device connected therewith, a stabilizing circuit coupled to said tunedcircuit through a capacity coupling having a high dielectric hysteresis constant, and an inductance connected in'said stabilizing circuit and provided with coils which are spaced apart by a dielectric of the same character as the dielectric of said coupling, said inductance being magnetically coupled with the tuned circuit in opposition to said capacity coupling.

6. In a tuned radio frequency amplifier comprising a. tuned circuit and a space discharge device connected therewith, the combination of a stabilizing circuit including an inductance having a natural frequency response which is higher than the highest op erating frequency of said tuned circuit, and

I an electrostatic coupling between said stabilizing circuit and the high potential end of said tuned circuit.

7. In a tuned radio frequency amplifier comprising a tuned circuit and a space discharge device connected therewith, the combination of a stabilizing circuit including an inductance having a natural frequency response which is higher than the highest operating frequency of said tuned circuit, and an electrostatic coupling between said stabilizing circuit and said tunedcircuit, said coupling including a dielectric having a high hysteresis loss constant. V 8. The combination with a tuned radio frequency amplifier comprising atuned circuit and a space discharge device connected therewith, of aninduction stabilizing circuit the natural frequency response period of which is higher than the highest operating frequency of the tuned circuit, and a'dielectric having a high hysteresis constant providing an electrostatic coupling betweensai circuits.

9. The combination of a tuned radio fre- 15 quency amplifier comprising a space discharge device and a tuned circuit connected therewith on which said device reacts to provide the effect of a negative resistance, and means including capacity and inductive cou- 20 plings for grading the positive resistance of the tuned circuit to neutralize said negative resistance, said capacity coupling being connected to the high potential end of saidtuned circuit and said inductive coupling being 25 connected to the low potential end of said tuned circuit.

10. The combination of a tuned radio frequency amplifier comprising a space discharge device and a tunedc-ircuit connected 3o therewith on which said device reacts to provide theeffect of a negative resistance, and a neutralizing circuit including a capacity and a coil for grading the positive resistance of the tuned circuit to neutralize said negative resistance, said neutralizing circuit being coupled to said tuned circuit through said capacity and through said coil, and said coil being arranged to neutralize the effect of said capacity at the lowest operating frequency -49 of said tuned circuit. 7

' In witness whereof, I have hereunto set my hand this 8th day of May, 1928.

ERNST F. W. ALEXANDERSON. 

